Method and system for measuring thermal stability factor of magnetic tunnel junction device, semiconductor integrated circuit, and production management method for semiconductor integrated circuit

ABSTRACT

A method and a system for measuring the thermal stability factor of a magnetic tunnel junction device, a semiconductor integrated circuit, and a production management method for the semiconductor integrated circuit, capable of measuring the thermal stability factors of individual devices in a relatively short period of time and quickly performing quality control during material development and at a production site. A meter measures change in resistance value of an evaluation MTJ for a predetermined period while causing a predetermined current to flow into the evaluation MTJ maintained at a predetermined temperature. An analyzer calculates a time constant in which a low-resistance state is maintained and a time constant in which a high-resistance state is maintained from the measured change in resistance value. A thermal stability factor of the evaluation MTJ is calculated on the basis of the calculated time constants and the predetermined current flowing into the evaluation MTJ.

FIELD OF THE INVENTION

The present invention relates to a method and a system for measuring athermal stability factor of a magnetic tunnel junction device, asemiconductor integrated circuit, and a production management method forthe semiconductor integrated circuit.

DESCRIPTION OF RELATED ART

A thermal stability factor is known as a factor indicating the thermalstability of bit information recorded in a nonvolatile memory such asMRAM having magneto-resistive devices. This thermal stability factor Δ₀is expressed by the following equation.Δ₀ =E _(b) /k _(B) T  (1)Here, E_(b) is an energy barrier necessary for magnetization reversal,k_(B) is the Boltzmann constant, and T is the absolute temperature.Here, the energy barrier is expressed by the following equation.E _(b) =K _(eff) V  (2)Here, K_(eff) is a magnetic anisotropy energy density of a recordinglayer and V is the volume of a recording layer.

The energy of a recording layer of a magneto-resistive device isexpressed by K_(eff)V·sin² θ (here, θ is an angle between amagnetization direction of a recording layer and a magnetizationdirection of a reference layer). When sin² θ=1 (θ=90°, 270°, . . . ), anenergy barrier (E_(b)) necessary for magnetization reversal is obtained.

A probability P that a magnetization of a recording layer having athermal stability factor Δ₀ is reversed after a certain time t isexpressed by the following equation by the Neel-Arrehenius Law (forexample, see Non-Patent Literature 1).P=1−exp{(−t/10⁻⁹)×exp(−Δ₀)}  (3)The time t when P is 50% is a retention period of the recording layer.

A magnetic field pulse method and a current pulse method are known asconventional general methods for measuring the thermal stability factorΔ₀. According to the magnetic field pulse method, a magnetizationreversal probability when a magnetic field pulse of a specific pulsewidth is applied is measured while changing the magnitude of a magneticfield of the magnetic field pulse, and the thermal stability factor Δ₀is calculated on the basis of the relation between the magnitude of themagnetic field and the reversal probability (for example, see Non-PatentLiterature 2). Moreover, according to the current pulse method, amagnetization reversal probability when a current pulse of a specificpulse width is applied is measured while changing the magnitude of acurrent of the current pulse, and the thermal stability factor Δ₀ iscalculated on the basis of the relation between the magnitude of thecurrent and the reversal probability (for example, see Non-PatentLiterature 3).

However, a magnetization reversal mode changes to a domain wall modewhen the diameter d of a recording layer is larger than a criticaldiameter dc and changes to a coherent reversal mode when the diameter dis smaller than the critical diameter dc. In the conventional magneticfield pulse method, there is a problem that, since analysis is performedusing the equation of the coherent reversal mode as the equation of theenergy barrier E_(b), the value of the energy barrier E_(b) in thedomain wall mode is smaller than an actual value, and an accuratethermal stability factor Δ₀ is not obtained (for example, see Non-PatentLiterature 4). It is considered that this problem occurs since the valueof the energy barrier E_(b) decreases due to the influence of an appliedmagnetic field when the magnetic field pulse method is performed. Asimilar problem may occur in the current pulse method. In recent years,in order to correct this, a method of performing analysis using theequation of E_(b) that is valid for the domain wall mode has beenproposed (for example, see Non-Patent Literature 5). Although it isconsidered that a correct value of E_(b) is obtained in the magneticfield pulse method if this equation is used, there is a problem that itis necessary to perform fitting using different equations for thecoherent reversal mode and the domain wall mode. Moreover, in themagnetic field pulse method, there is a problem that, since the responseof an electromagnet for generating a magnetic field is slow, measurementtakes a considerable amount of time, and several tens of minutes toseveral hours are taken for measuring the thermal stability factor Δ₀ ofone device.

Therefore, as a method for measuring the thermal stability factor Δ₀different from the magnetic field pulse method and the current pulsemethod, a method in which an MRAM chip made up of MTJs of 10 Mb orhigher is prepared, information of 1/0 is written in the recording layerof each MTJ using a checker board pattern or the like, the MRAM chiprests at a high temperature for several minutes to approximately 100hours, an error rate indicating how much of the initial information hasdisappeared is measured, and the thermal stability factor Δ₀ iscalculated on the basis of a relation between the resting time and areversal probability calculated from the error rate (for example, seeNon-Patent Literature 6). According to this method, since the value ofthe energy barrier E_(b) does not change during measurement, it ispossible to calculate an accurate thermal stability factor Δ₀ regardlessof the magnetization reversal mode.

CITATION LIST Non-Patent Literature

-   Non-Patent Literature 1: W. F. Brown, Jr., “Thermal Fluctuations of    a Single-Domain Particle”, Phys. Rev., 1963, Vol. 130, Num. 5, p.    1677-1686-   Non-Patent Literature 2: H. Sato, M. Yamanouchi, S. Ikeda, S.    Fukami, F. Matsukura, and H. Ohno, “Perpendicular-anisotropy    CoFeB—MgO magnetic tunnel junctions with a MgO/CoFeB/Ta/CoFeB/MgO    recording structure”, Appl. Phys. Lett., 2012, 101, 022414-   Non-Patent Literature 3: R. H. Koch, J. A. Katine, and J. Z. Sun,    “Time-Resolved Reversal of Spin-Transfer Switching in a Nanomagnet”,    Phys. Rev. Lett., 2004, Vol. 92, Num. 8, 088302-   Non-Patent Literature 4:Gabriel D. Chaves-O'Flynn, Georg Wolf,    Jonathan Z. Sun, and Andrew D. Kent, “Thermal Stability of Magnetic    States in Circular Thin-Film Nanomagnets with Large Perpendicular    Magnetic Anisotropy”, Phys. Rev. Appl., 2015, 4, 024010-   Non-Patent Literature 5: Luc Thomas, Guenole Jan, Son Le, Yuan-Jen    Lee, Huanlong Liu, Jian Zhu, Santiago Serrano-Guisan, Ru-Ying Tong,    Keyu Pi, Dongna Shen, Renren He, Jesmin Haq, Zhongjian Teng, Rao    Annapragada, Vinh Lam, Yu-Jen Wang, Tom Zhong, Terry Torng and    Po-Kang Wang, “Solving the Paradox of the Inconsistent Size    Dependence of Thermal Stability at Device and Chip-level in    Perpendicular STT-MRAM”, Proc IEDM2015, 672-   Non-Patent Literature 6: Luc Thomas, Guenole Jan, Son Le, and    Po-Kang Wang, “Quantifying data retention of perpendicular    spin-transfer-torque magnetic random access memory chips using an    effective thermal stability factor method”, Appl. Phys. Lett., 2015,    106, 162402

SUMMARY OF THE INVENTION

The thermal stability factor measurement method disclosed in Non-PatentLiterature 6 measures a thermal stability factor of a chip but does notmeasure the thermal stability factor of individual devices such as MTJs.Moreover, although an MRAM chip having a capacity of several Mb orhigher is necessary in order to perform measurement, since approximatelythree months are taken for fabrication of the chip, it takes aconsiderable amount of time until data is obtained, and the wastefultime increases when the chip is used for quality control (QC checking)during material development and at a production site. Moreover, themeasurement time increases as the thermal stability factor increases.

The present invention has been made in view of such a problem, and anobject thereof is to provide a method and a system for measuring thethermal stability factor of a magnetic tunnel junction device, asemiconductor integrated circuit, and a production management method forthe semiconductor integrated circuit, capable of measuring the thermalstability factors of individual devices in a relatively short period oftime and quickly performing quality control during material developmentand at a production site.

In order to attain the object, a method for measuring a thermalstability factor of a magnetic tunnel junction device according to thepresent invention includes: a measurement step of measuring change inresistance value of the magnetic tunnel junction device for apredetermined period while causing a predetermined current to flow intothe magnetic tunnel junction device maintained at a predeterminedtemperature; a first calculation step of calculating a time constant inwhich a low-resistance state is maintained and a time constant in whicha high-resistance state is maintained from the change in resistancevalue measured in the measurement step; and a second calculation step ofcalculating a thermal stability factor of the magnetic tunnel junctiondevice on the basis of the predetermined current and the time constantscalculated in the first calculation step.

A system for measuring a thermal stability factor of a magnetic tunneljunction device according to the present invention includes: atemperature controller that maintains the magnetic tunnel junctiondevice at a predetermined temperature; a meter that measures change inresistance value of the magnetic tunnel junction device for apredetermined period while causing a predetermined current to flow intothe magnetic tunnel junction device; and an analyzer that calculates atime constant in which a low-resistance state is maintained and a timeconstant in which a high-resistance state is maintained from the changein resistance value measured by the meter and calculates a thermalstability factor of the magnetic tunnel junction device on the basis ofthe time constants and the predetermined current.

According to the method and the system for measuring the thermalstability factor of the magnetic tunnel junction device according to thepresent invention, it is not necessary to generate a magnetic fieldduring measurement and measurement can be performed on respectivedevices rather than the entire chip having a number of devices.Therefore, it is possible to measure the thermal stability factor in arelatively short period of time. Moreover, since the value of the energybarrier E_(b) does not change during measurement like the magnetic fieldpulse method and the current pulse method, it is possible to calculatean accurate thermal stability factor. The method and the system formeasuring the thermal stability factor of the magnetic tunnel junctiondevice according to the present invention may perform measurement on asingle device only and may perform measurement on a single device aplurality of times, and may perform measurement on a plurality ofdevices collectively.

According to the method and the system for measuring the thermalstability factor of the magnetic tunnel junction device according to thepresent invention, since the thermal stability factor can be measuredfor respective magnetic tunnel junction devices, it is not necessary tofeed the management information based on the measurement result back tothe initial stage of the chip manufacturing process, but the managementinformation may be fed back to the stage of manufacturing the magnetictunnel junction device. Due to this, it is possible to save a periodfrom the start of a chip manufacturing process to the preceding stage ofmanufacturing the magnetic tunnel junction device and to quickly performquality control during material development and at a production site ascompared to a case of feeding the management information back to theinitial stage of the chip manufacturing process. In the method and thesystem for measuring the magnetic tunnel junction device according tothe present invention, it is preferable that the change in resistancevalue of the magnetic tunnel junction device is measured using a prober.

In the method for measuring the thermal stability factor of the magnetictunnel junction device according to the present invention, it ispreferable that the measurement step involves measuring the change inresistance value for each of a plurality of currents of differentmagnitudes while supplying the plurality of currents sequentially as thepredetermined current, the first calculation step involves calculating atime constant τ_(P) in which the low-resistance state is maintained anda time constant τ_(AP) in which the high-resistance state is maintainedfor each of the currents, and the second calculation step involvescalculating the thermal stability factor on the basis of the magnitudeof each of the currents and the time constants τ_(P) and τ_(AP)corresponding to each of the currents. In the system for measuring thethermal stability factor of the magnetic tunnel junction deviceaccording to the present invention, it is preferable that the meter isconfigured to measure the change in resistance value for each of aplurality of currents of different magnitudes while supplying theplurality of currents sequentially as the predetermined current, and theanalyzer is configured to calculate a time constant τp in which thelow-resistance state is maintained and a time constant τ_(AP) in whichthe high-resistance state is maintained for each of the currents andcalculate the thermal stability factor on the basis of the magnitude ofeach of the currents and the time constants τ_(P) and τ_(AP)corresponding to each of the currents. In this case, it is possible tocalculate the thermal stability factor more accurately.

In the method for measuring the thermal stability factor of the magnetictunnel junction device according to the present invention, it ispreferable that the first calculation step involves calculating afrequency distribution N_(P)(t) of a period in which the low-resistancestate is maintained and a frequency distribution N_(Δp)(t) of a periodin which the high-resistance state is maintained from the change inresistance value measured in the measurement step and calculating thetime constant τ_(P) in which the low-resistance state is maintained andthe time constant τ_(AP) in which the high-resistance state ismaintained using relations of N_(P)(t)∝ exp(−t/τ_(P)) and N_(AP)(t)∝exp(−t/τ_(AP)). In the system for measuring the thermal stability factorof the magnetic tunnel junction device according to the presentinvention, it is preferable that the analyzer is configured to calculatethe frequency distribution N_(P)(t) of the period in which thelow-resistance state is maintained and the frequency distributionN_(AP)(t) of the period in which the high-resistance state is maintainedfrom the change in resistance value measured in the measurement step andcalculate the time constant τ_(P) in which the low-resistance state ismaintained and the time constant τ_(AP) in which the high-resistancestate is maintained using relations of N_(P)(t)∝ exp(−t/τ_(P)) andN_(AP)(t)∝ exp(−t/τ_(AP)). In this case, it is possible to calculate thethermal stability factor more accurately.

In the method for measuring the thermal stability factor of the magnetictunnel junction device according to the present invention, it ispreferable that the second calculation step involves calculating athermal stability factor Δ₀ and a threshold current value I_(c0), on thebasis of the predetermined current I and the time constant τ_(P) inwhich the low-resistance state is maintained and the time constantτ_(AP) in which the high-resistance state is maintained calculated inthe first calculation step, using the following equation.τ_(P)/(τ_(P)+τ_(AP))=1/[1+exp{−Δ₀·(2×I/I _(c0))}]  (4)In the system for measuring the thermal stability factor of the magnetictunnel junction device according to the present invention, it ispreferable that the analyzer is configured to calculate the thermalstability factor Δ₀ and the threshold current value I_(c0), on the basisof the predetermined current I and the time constant τ_(P) in which thelow-resistance state is maintained and the time constant τ_(AP) in whichthe high-resistance state is maintained calculated in the firstcalculation step, using Equation (4). In this case, it is possible tocalculate the thermal stability factor more accurately.

In the method for measuring the thermal stability factor of the magnetictunnel junction device according to the present invention, it ispreferable that the measurement step involves measuring the change inresistance value while causing the predetermined current to flow intothe magnetic tunnel junction device from a testing terminal which iselectrically connected to the magnetic tunnel junction device only sothat current flows into the magnetic tunnel junction device only. In thesystem for measuring the thermal stability factor of the magnetic tunneljunction device according to the present invention, it is preferablethat the system includes a testing terminal which is electricallyconnected to the magnetic tunnel junction device only so that currentflows into the magnetic tunnel junction device only, and the meter isconfigured to measure the change in resistance value while causing thepredetermined current to flow into the magnetic tunnel junction devicefrom the testing terminal. In this case, it is possible to calculate anaccurate thermal stability factor of a single magnetic tunnel junctiondevice using the testing terminal.

A semiconductor integrated circuit according to the present invention isa semiconductor integrated circuit including a plurality of magnetictunnel junction devices, the semiconductor integrated circuit including:a testing terminal which is electrically connected to a single magnetictunnel junction device so that current flows into the magnetic tunneljunction device only.

Since the semiconductor integrated circuit according to the presentinvention can supply a current to a single magnetic tunnel junctiondevice only using the testing terminal, it is possible to performmeasurement and examination using current with respect to the singlemagnetic tunnel junction device. For example, it is possible tocalculate an accurate thermal stability factor of the single magnetictunnel junction device using the method and the system for measuring thethermal stability factor of the magnetic tunnel junction deviceaccording to the present invention. The testing terminal may include apair of terminals, and the terminals may be electrically connected to asingle magnetic tunnel junction device only. The testing terminal ispreferably formed of an upper wiring or a lower wiring of the magnetictunnel junction device and a connection region (VIA).

In the semiconductor integrated circuit according to the presentinvention, it is preferable that the testing terminal is provided in onemagnetic tunnel junction device or in each of a plurality of magnetictunnel junction devices. In this case, it is possible to performmeasurement and examination with respect to one or a plurality ofmagnetic tunnel junction devices in which the testing terminal isprovided and to calculate the thermal stability factor and the like.

In the semiconductor integrated circuit according to the presentinvention, it is preferable that the testing terminal is used forsupplying the predetermined current and measuring the change inresistance value in the measurement step of the method for measuring thethermal stability factor of the magnetic tunnel junction deviceaccording to the present invention. In this case, it is possible tocalculate an accurate thermal stability factor of the magnetic tunneljunction device in which the testing terminal is provided.

A production management method for the semiconductor integrated circuitaccording to the present invention is a production management method fora semiconductor integrated circuit including a plurality of magnetictunnel junction devices, wherein, after the plurality of magnetic tunneljunction devices are manufactured, a thermal stability factor of one ora plurality of the plurality of magnetic tunnel junction devices iscalculated according to the method for measuring the thermal stabilityfactor of the magnetic tunnel junction device according to the presentinvention, and a process of manufacturing the plurality of magnetictunnel junction devices are managed on the basis of the calculatedthermal stability factor.

According to the production management method for the semiconductorintegrated circuit according to the present invention, since the thermalstability factor can be measured for respective magnetic tunnel junctiondevices by the method for measuring the thermal stability factor of themagnetic tunnel junction device according to the present invention, itis not necessary to feed the management information based on themeasurement result back to the initial stage of the chip manufacturingprocess, but the management information may be fed back to the stage ofmanufacturing the magnetic tunnel junction device. Due to this, it ispossible to save a period from the start of a chip manufacturing processto the preceding stage of manufacturing the magnetic tunnel junctiondevice and to quickly perform production management as compared to acase of feeding the management information back to the initial stage ofthe chip manufacturing process. In the production management method forthe semiconductor integrated circuit according to the present invention,it is preferable that the semiconductor integrated circuit is thesemiconductor integrated circuit according to the present invention.

According to the present invention, it is possible to provide a methodand a system for measuring the thermal stability factor of a magnetictunnel junction device, a semiconductor integrated circuit, and aproduction management method for the semiconductor integrated circuit,capable of measuring the thermal stability factors of individual devicesin a relatively short period of time and quickly performing qualitycontrol during material development and at a production site.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a semiconductor integratedcircuit according to an embodiment of the present invention.

FIG. 2 is a flowchart illustrating a process of manufacturing thesemiconductor integrated circuit according to the embodiment of thepresent invention.

FIG. 3 is a perspective view illustrating an evaluation MTJ of thesemiconductor integrated circuit according to the embodiment of thepresent invention and is a block diagram illustrating a system formeasuring a thermal stability factor of the magnetic tunnel junctiondevice according to the embodiment of the present invention.

FIG. 4(a) is a plan view in which evaluation MTJs are arranged atcorners of a memory chip of the semiconductor integrated circuitaccording to the embodiment of the present invention, and FIG. 4(b) is aplan view in which evaluation MTJs are arranged in respective sub-arraysin the memory chip.

FIG. 5 is a perspective view illustrating an MTJ array structure of thesemiconductor integrated circuit according to the embodiment of thepresent invention.

FIG. 6(a) illustrates an example of an observed waveform of a resistancevalue of an evaluation MTJ observed by an oscilloscope according to amethod for measuring the thermal stability factor of a magnetic tunneljunction device according to an embodiment of the present invention,FIG. 6(b) is a graph illustrating a frequency distribution of a periodin which a low-resistance state is maintained and a period in which ahigh-resistance state is maintained and an example of a curve obtainedby fitting, and FIG. 6(c) is a graph illustrating a relation between acurrent flowing into the evaluation MTJ and the time constants for thelow-resistance state and the high-resistance state and an example of acurve obtained by fitting.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings.

FIGS. 1 to 6 illustrate a method and a system for measuring a thermalstability factor of a magnetic tunnel junction device, a semiconductorintegrated circuit, and a production management method for thesemiconductor integrated circuit according to an embodiment of thepresent invention.

Semiconductor Integrated Circuit Manufacturing Process

As illustrated in FIG. 1, a semiconductor integrated circuit 10 of anembodiment of the present invention is formed of an MRAM(magnetoresistance memory) having a number of MTJs (magnetic tunneljunction devices) 11 and is manufactured according to a manufacturingprocess illustrated in FIG. 2.

As illustrated in FIGS. 1 and 2, first, a wafer 12 serving as asubstrate is loaded (step 21), a CMOS 13 is fabricated on a surfacethereof (step 22), and a multi-layered intermediate wiring 14 (forexample, M1-M4 wiring) for connection to the MTJs 11 is formed on thefabricated CMOS 13 (step 23). Generally, the manufacturing process up tothis step 23 takes approximately two months. Subsequently, a magneticfilm is formed by PVD (physical vapor deposition) on an upper part ofthe intermediate wiring 14 (step 24), and a film property of themagnetic film is examined (step 25). After that, an MTJ pattern isformed by lithography (step 26) and the shape of the lithography isexamined (step 27). After that, the MTJs 11 are formed by etching (step28), and the shape of the MTJs 11 is examined (step 29). Generally, theMTJ manufacturing process of steps 24 to 29 takes approximately oneweek.

After the shape of the MTJs 11 is examined, an upper wiring 15 is formedon the MTJs 11 (step 30). Generally, this step 30 ends in two weeks.After the upper wiring 15 is formed, electrical characteristics ofsingle MTJs 11 are examined (step 31), and the characteristics of an MTJarray are examined (step 32). Examples of the examined electricalcharacteristics of the MTJs 11 include a resistance value, thermalstability, a writing current, an error rate, and the like. Examples ofthe examined characteristics of the MTJ array include a bit error rate,a resistance distribution, and the like.

In step 30, as illustrated in FIG. 3, a testing terminal block 17, whichincludes the intermediate wiring 14, a connection region (VIA) 16, andthe upper wiring 15, is formed so as to be electrically connected to anevaluation MTJ 11 a only without being electrically connected to theCMOS 13 illustrated in FIG. 1 or the intermediate wiring 14 connected tothe CMOS 13. In the example illustrated in FIG. 3, the testing terminalblock 17 is made up of a pair of terminals. One terminal is formed ofthe upper wiring 15 formed in an upper part of a single evaluation MTJ11 a, and the other terminal includes an intermediate wiring (BASE) 14connected to a lower part of the evaluation MTJ 11 a, the connectionregion 16 formed on the intermediate wiring 14, and the upper wiring 15.The testing terminal block 17 is configured to supply current to onlythe single evaluation MTJ lla electrically connected thereto and can beused when examining the electrical characteristics in step 31.

One to several testing terminal blocks 17 may be disposed at a corner ofa memory chip as a single TEG (test element group) as illustrated inFIG. 4(a), and one to several testing terminal blocks 17 may be arrangedin respective sub-arrays in the memory chip as illustrated in FIG. 4(b).The disposed one to several testing terminal blocks 17 have a pad 17 aconnected to the upper wiring 15, which is completely independent from apad group for realizing external electrical connection to bit lines andword lines of a memory array to be described later. A probe such as aprober for realizing electrical connection to various electricalmeasurement apparatuses, for example, comes into contact with this pad17 a to electrically measure the characteristics of the evaluation MTJ11 a. On the other hand, the pad group for realizing external electricalconnection to bit lines and word lines of a memory array is connected topins of a package via wires, for example. Therefore, it is not possibleto measure the electrical characteristics of the single evaluation MTJ11 a using these pads.

In this manner, the semiconductor integrated circuit 10 made up of MRAMmemory chips can be manufactured according to the manufacturing methodillustrated in FIG. 2.

As illustrated in FIG. 5, the manufactured semiconductor integratedcircuit 10 excluding the evaluation MTJ 11 a is formed of MRAMs in whicha plurality of bit lines 18 are wired as the upper wiring 15 of the MTJs11 and a plurality of word lines 19 are wired in the upper part of theCMOS 13. The bit lines 18 are arranged in parallel to each other and areconnected to a bit-line selection circuit 18 a. The word lines 19 arearranged in parallel to each other and are connected to a word-lineselection circuit 19 a.

In this semiconductor integrated circuit 10, when data is written to anarbitrary MTJ 11, the bit-line selection circuit 18 a and the word-lineselection circuit 19 a apply a voltage to a predetermined bit line 18and a predetermined word line 19, respectively, on the basis of awriting bit number. Moreover, when data is read from an arbitrary MTJ11, the bit-line selection circuit 18 a and the word-line selectioncircuit 19 a select a predetermined bit line 18 and a predetermined wordline 19, respectively, on the basis of a reading bit number and connectthe selected bit line and word line to a sense amplifier 20. Theexamination of the characteristics of the MTJ array in step 32 of FIG. 2can be performed using the circuit illustrated in FIG. 5.

Measurement of Thermal Stability Factor

Next, a method and a system for measuring the thermal stability factorof the magnetic tunnel junction device according to the embodiment ofthe present invention, for measuring the thermal stability factor amongthe electrical characteristics of the single evaluation MTJ 11 aexamined in step 31 of FIG. 2 will be described.

As illustrated in FIG. 3, a system 40 for measuring the thermalstability factor of the magnetic tunnel junction device according to theembodiment of the present invention includes a temperature controller(not illustrated), a meter 41, and an analyzer 42.

The temperature controller is formed of a prober capable of controllingtemperature and has a wafer 12 including the evaluation MTJ 11 a mountedthereon so that the evaluation MTJ can be maintained at a predeterminedtemperature. The meter 41 includes a pair of probes 41 a of the proberof the temperature controller, a voltage pulse generator 41 b, areference resistance 41 c, and a voltage meter 41 d, for example. Thevoltage pulse generator 41 b is connected to the respective probes 41 ato apply a voltage pulse between the probes 41 a. The referenceresistance 41 c is connected in series between one probe 41 a and thevoltage pulse generator 41 b. The voltage meter 41 d is formed of ameter such as an oscilloscope capable of measuring a voltage and isconnected in parallel to the reference 41 c to measure a voltagegenerated in the reference resistance 41 c. For example, a DC currentsource and a trigger mechanism for designating a timing for generating acurrent from the DC current source may be used instead of the voltagepulse generator 41 b.

The meter 41 is configured such that the probes 41 a are brought intocontact with the testing terminal blocks 17 of the evaluation MTJ 11 a,a predetermined voltage pulse is generated by the voltage pulsegenerator 41 b and is applied across both ends of the evaluation MTJ 11a to cause a predetermined current to flow into the evaluation MTJ 11 a.Moreover, a voltage generated between both ends of the referenceresistance 41 c generated due to the current is monitored and measuredby the voltage meter 41 d for a predetermined period to measure changein resistance value of the evaluation MTJ 11 a.

The analyzer 42 is formed of a computer and is connected to the voltagemeter 41 d to receive the measured values. The analyzer 42 calculates atime constant in which a low-resistance state is maintained and a timeconstant in which a high-resistance state is maintained from the changein resistance value measured by the voltage meter 41 d. Moreover, theanalyzer 42 calculates a thermal stability factor of the evaluation MTJ11 a on the basis of the calculated time constants and the currentflowing into the evaluation MTJ 11 a.

The method for measuring the thermal stability factor of the magnetictunnel junction device according to the embodiment of the presentinvention can be ideally performed by the system 40 for measuring thethermal stability factor of the magnetic tunnel junction deviceillustrated in FIG. 3. That is, in the method for measuring the thermalstability factor of the magnetic tunnel junction device according to theembodiment of the present invention, first, in a state in which theevaluation MTJ 11 a is maintained at a predetermined temperature by thetemperature controller, the meter 41 measures change in resistance valueof the evaluation MTJ 11 a for a predetermined period while causing apredetermined current to flow into the evaluation MTJ 11 a. A specificmeasurement example in the voltage meter 41 a formed of an oscilloscopein this case is illustrated in FIG. 6(a). A measurement temperature was240° C. The measurement period in FIG. 6(a) was 500 msec. As illustratedin FIG. 6(a), it can be ascertained that the measured change inresistance value includes a low-resistance state and a high-resistancestate.

Subsequently, the analyzer 42 calculates a frequency distributionN_(P)(t) of a period in which a low-resistance state is maintained and afrequency distribution N_(AP)(t) of a period in which a high-resistancestate is maintained from the measured change in resistance value andcalculates a time constant τ_(P) in which a low-resistance state ismaintained and a time constant τ_(AP) in which a high-resistance stateis maintained from the decaying states of the frequency distributionsusing the relations of N_(P)(t)∝ exp(−t,τ_(P)) and N_(Δp)(t)∝exp(−t/τ_(AP)), respectively, by fitting such as the least-squaresmethod. The frequency distributions N_(P)(t) and N_(AP)(t) of the periodin which a low-resistance state is maintained and the period in which ahigh-resistance state is maintained obtained by actual measurement andan example of a curve obtained by fitting are illustrated in FIG. 6(b).

The meter 41 measures change in resistance value for each of a pluralityof currents of different magnitudes while supplying the plurality ofcurrents sequentially and the analyzer 42 calculates the time constantτ_(P) in which the low-resistance state is maintained and the timeconstant τ_(AP) in which the high-resistance state is maintained,corresponding to each of the currents.

After the time constants τ_(P) and τ_(AP) for the plurality of currentsof different magnitudes are calculated, a thermal stability factor Δ₀and a threshold current value I_(c0) are calculated by fitting such asthe least-squares method using Equation (4). A relation between eachcurrent and the left side τ_(P)/(τ_(P)+τ_(AP)) of Equation (4) obtainedby actual measurement and an example of the curve obtained by fittingare illustrated in FIG. 6(c). In the specific example illustrated inFIG. 6(c), the thermal stability factor Δ₀ was 24.69 and the thresholdcurrent value I_(c0) was 175.44 μA when the temperature of theevaluation MTJ 11 a was 240° C. The thermal stability factor at a roomtemperature is estimated to be 24.69×(240+273)/(25+273)=43.5. In thismanner, it is possible to calculate the thermal stability factor at alloperating temperatures including a room temperature easily frommeasurements performed at high temperatures.

In the embodiment, an example in which the change in resistance valuefor each of a plurality of currents of different magnitudes is measuredwhile supplying the plurality of currents sequentially, and the analyzer42 calculates the time constant τ_(P) in which a low-resistance state ismaintained and the time constant τ_(AP) in which a high-resistance stateis maintained, corresponding to each of the currents, has beendescribed. However, a DC current sufficient for monitoring a resistancestate may be used as the current, the time constant τ_(P) in which thelow-resistance state is maintained and the time constant τ_(AP) in whichthe high-resistance state is maintained may be calculated for eachmagnetic field while changing a magnetic field pulse (intensity H), andthe thermal stability factor Δ₀ and a threshold magnetic field H_(C0)may be calculated using Equation (5) below, for example.τ_(P)/(τ_(P)+τ_(AP))=1/[1+exp{−Δ₀·(4×H/H _(C0))}]  (5)

In the measurement method which uses the current or the magnetic field,the temperature may be changed while fixing the current or magneticfield value. In this case, by changing the value of the temperature T inΔ₀=E_(b)/k_(B)T of Equation (4) or (5), E_(b) can be calculated from themeasurement result of different T.

As described above, according to the method and the system 40 formeasuring the thermal stability factor of the magnetic tunnel junctiondevice according to the embodiment of the present invention, it is notnecessary to generate a magnetic field during measurement andmeasurement is performed on the single evaluation MTJ 11 a only ratherthan the entire chip having a number of devices. Therefore, it ispossible to measure the thermal stability factor Δ₀ of the singleevaluation MTJ 11 a in a relatively short period of time. Moreover,since the value of the energy barrier E_(b) does not change duringmeasurement like the magnetic field pulse method and the current pulsemethod, it is possible to calculate an accurate thermal stability factorΔ₀ of the single evaluation MTJ 11 a.

Production Management of Semiconductor Integrated Circuit

Next, a production management method for the semiconductor integratedcircuit according to the embodiment of the present invention will bedescribed.

When the thermal stability factor of one or a plurality of evaluationMTJs 11 a on the memory chip is calculated by the method and the system40 for measuring the thermal stability factor of the magnetic tunneljunction device according to the embodiment of the present invention instep 31 of FIG. 2, management of the MTJ manufacturing process of steps24 to 29 is performed on the basis of the thermal stability factor. Forexample, when a desired thermal stability factor is not obtained, thethermal stability factor may be fed back to step 24 and the conditionsfor manufacturing the MTJ 11 may be changed.

According to the production management method for the semiconductorintegrated circuit according to the embodiment of the present invention,it is not necessary to feed the management information based on themeasurement result of the evaluation MTJ 11 a back to the initial stage(step 21) of the chip manufacturing process. Due to this, it is possibleto save a period from the start of the chip manufacturing process to thepreceding stage (steps 21 to 23) of manufacturing the MTJ 11 as comparedto a case of feeding the management information back to the initialstage of the chip manufacturing process. For example, in themanufacturing process illustrated in FIG. 2, it is possible to save twomonths for steps 21 to 23 and quickly perform quality control andproduction management during material development and at a productionsite.

REFERENCE SIGNS LIST

-   10: Semiconductor integrated circuit-   11: MTJ-   12: Wafer-   13: CMOS-   14: Intermediate wiring-   15: Upper wiring-   16: Connection region-   17: Testing terminal block    -   17 a: Pad-   18: Bit line-   18 a: Bit-line selection circuit-   19: Word line-   19 a: Word-line selection circuit-   20: Sense amplifier-   40: System for measuring thermal stability factor of magnetic tunnel    junction device-   41: Meter    -   41 a: Probe    -   41 b: Voltage pulse generator    -   41 c: Reference    -   41 d: Voltage meter-   42: Analyzer

What is claimed is:
 1. A method for measuring a thermal stability factorof a magnetic tunnel junction device, comprising: a measurement step ofmeasuring change in resistance value of the magnetic tunnel junctiondevice for a predetermined period while causing a predetermined currentto flow into the magnetic tunnel junction device maintained at apredetermined temperature; a first calculation step of calculating atime constant in which a low-resistance state is maintained and a timeconstant in which a high-resistance state is maintained from the changein resistance value measured in the measurement step, the firstcalculation step comprising calculating a frequency distributionN_(P)(t1) of a period t1 in which the low-resistance state is maintainedand a frequency distribution N_(AP)(t2) of a period t2 in which thehigh-resistance state is maintained from the change in resistance valuemeasured in the measurement step and calculating the time constant τ_(P)in which the low-resistance state is maintained and the time constantτ_(AP) in which the high-resistance state is maintained using relationsof N_(P)(t1) ∝ exp(−t1/τ_(P)) and N_(AP)(t2) ∝ exp(−t2/τ_(AP)),respectively; and a second calculation step of calculating a thermalstability factor of the magnetic tunnel junction device based on thepredetermined current and the time constants calculated in the firstcalculation step.
 2. The method for measuring the thermal stabilityfactor of the magnetic tunnel junction device according to claim 1,wherein the measurement step comprises measuring the change inresistance value for each of a plurality of currents of differentmagnitudes while supplying the plurality of currents sequentially as thepredetermined current, the first calculation step comprises calculatingthe time constant τ_(P) in which the low-resistance state is maintainedand the time constant τ_(AP) in which the high-resistance state ismaintained for each of the currents, the second calculation stepcomprises calculating the thermal stability factor based on themagnitude of each of the currents and the time constants τ_(P) andτ_(AP) corresponding to each of the currents.
 3. The method formeasuring the thermal stability factor of the magnetic tunnel junctiondevice according to claim 1, wherein the second calculation stepcomprises calculating a thermal stability factor Δ₀ and a thresholdcurrent value I_(c0), based on the predetermined current I and the timeconstant τ_(P) in which the low-resistance state is maintained and thetime constant τ_(AP) in which the high-resistance state is maintainedcalculated in the first calculation step, using an equation ofτ_(P)/(τ_(P)±τ_(AP))=1/[1+exp{−Δ₀·(2×I/I_(c0))}].
 4. The method formeasuring the thermal stability factor of the magnetic tunnel junctiondevice according to claim 1, wherein the measurement step comprisesmeasuring the change in resistance value while causing the predeterminedcurrent to flow into the magnetic tunnel junction device from a testingterminal which is electrically connected to the magnetic tunnel junctiondevice only so that current flows into the magnetic tunnel junctiondevice only.
 5. A production management method for a semiconductorintegrated circuit including a plurality of magnetic tunnel junctiondevices, comprising calculating a thermal stability factor of one or aplurality of the plurality of magnetic tunnel junction devices accordingto the method for measuring the thermal stability factor of the magnetictunnel junction device according to claim 1; and managing a process ofmanufacturing the plurality of magnetic tunnel junction devices based onthe calculated thermal stability factor.
 6. The production managementmethod for the semiconductor integrated circuit according to claim 5,wherein the semiconductor integrated circuit comprises: a testingterminal which is electrically connected to a single magnetic tunneljunction device so that current flows into the single magnetic tunneljunction device only.
 7. A system for measuring a thermal stabilityfactor of a magnetic tunnel junction device, comprising: a temperaturecontroller that maintains the magnetic tunnel junction device at apredetermined temperature; a meter that measures change in resistancevalue of the magnetic tunnel junction device for a predetermined periodwhile causing a predetermined current to flow into the magnetic tunneljunction device; and an analyzer configured to: calculate a timeconstant in which a low-resistance state is maintained and a timeconstant in which a high-resistance state is maintained from the changein resistance value measured by the meter and calculates a thermalstability factor of the magnetic tunnel junction device based on thetime constants and the predetermined current; and calculate a frequencydistribution N_(P)(t1) of a period t1 in which a low-resistance state ismaintained and a frequency distribution N_(AP)(t2) of a period t2 inwhich a high-resistance state is maintained from the change inresistance value measured by the meter and calculate the time constantτ_(P) in which the low-resistance state is maintained and the timeconstant τ_(AP) in which the high-resistance state is maintained usingrelations of N_(P)(t1)∝ exp(−t1/τ_(P)) and N_(AP)(t2)∝ exp(t2/τ_(AP)),respectively.
 8. The system for measuring the thermal stability factorof the magnetic tunnel junction device according to claim 7, wherein themeter is configured to measure the change in resistance value for eachof a plurality of currents of different magnitudes while supplying theplurality of currents sequentially as the predetermined current, and theanalyzer is configured to calculate the time constant τ_(P) in which thelow-resistance state is maintained and the time constant τ_(AP) in whichthe high-resistance state is maintained for each of the currents andcalculate the thermal stability factor based on the magnitude of each ofthe currents and the time constants τ_(P) and τ_(AP) corresponding toeach of the currents.
 9. The system for measuring the thermal stabilityfactor of the magnetic tunnel junction device according to claim 7,wherein the analyzer is configured to calculate a thermal stabilityfactor Δ₀ and a threshold current value I_(c0), based on thepredetermined current I and the time constant τ_(P) in which thelow-resistance state is maintained and the time constant τ_(AP) in whichthe high-resistance state is maintained calculated in the firstcalculation step, using an equation ofτ_(P)/(τ_(P)+τ_(AP))=1/[1+exp{−Δ₀·(2×I/I_(c0))}].
 10. The system formeasuring the thermal stability factor of the magnetic tunnel junctiondevice according to claim 7, wherein the system comprises a testingterminal which is electrically connected to the magnetic tunnel junctiondevice only so that current flows into the magnetic tunnel junctiondevice only, and the meter is configured to measure the change inresistance value while causing the predetermined current to flow intothe magnetic tunnel junction device from the testing terminal.